The information watchdog is to launch an investigation after a list apparently containing the personal details of thousands of members of the British National Party (BNP) was leaked onto the internet for the second time in less than a year.

Nick Griffin, the party’s leader, branded the leak a forgery today, adding that it had been timed to cause the maximum amount of damage ahead of his debut on BBC1’s Question Time on Thursday. The names, addresses and telephone numbers of almost 12,000 people were revealed in the document, which was posted on the internet site WikiLeaks.

Mr Griffin’s fellow Question Time panellists have been plotting ways to expose the far-right party’s leader during the broadcast. One member of the panel told The Independent that they would be taking a dossier onto the programme with them, detailing alleged anti-Semitic and racist comments reportedly made by Mr Griffin and asking him to explain them. “I have never gone on the programme with notes before,” the panellist said. “But this time I will have details of dates, places and quotes.”

Several doctors and members of the military appeared on the alleged membership document. It appeared to show a slight fall in membership since a similar list surfaced last November. The Information Commissioner’s Office (ICO) confirmed it was examining the data leak. Mick Gorrill, the assistant commissioner, said: “We will establish the full facts before deciding on any regulatory action.”

Writing on his party’s website today, Mr Giffin said that the list was a forgery. “It is a concoction of the ‘old’ list plus a number of inquiries received, but, most disturbingly, it contains thousands of names of people with whom the BNP has had no contact whatsoever,” he said. “We have no idea from where this information has been drawn. Some of it looks like random items drawn from a telephone book.”

The BBC Trust could yet enforce a last-minute reversal of the decision to invite Mr Griffin onto Question Time after Peter Hain, the Welsh Secretary, made a final plea to the corporation’s governance body to block his participation. Mark Thompson, the BBC’s director general, has already rejected Mr Hain’s claim that the corporation must not give airtime to the far-right party on legal grounds.

“I have a legitimate expectation that the Trust will not avoid this difficult decision for its own political convenience,” Mr Hain writes in his letter to the trust. “This is a fast changing situation. The BBC must not hide behind the process and procedure to usher the BNP across this threshold.”

Anti-fascist demonstrators are to hold a press conference outside the BBC’s Television Centre, West London, on Thursday morning to mark the start of a day-long protest against Mr Griffin’s Question Time appearance. Dozens of BBC workers are set to join them as they finish work. A rally organised by Unite Against Fascism will also be held tonight in West London, during which Mr Hain and Michael Rosen, the former children’s laureate, will be addressing the crowd.

Security on the programme has been stepped up to prevent protesters forming part of the Question Time audience. It is understood that all potential audience members will be subjected to extra vetting before they are accepted. They will be asked which political party they will be supporting at the next election as the BBC attempts to ensure a range of views, including some sympathetic to the BNP. Thursday’s programme is also being filmed earlier than usual to ensure that producers have enough time to complete the show should any disturbances take place.

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